Hi all, I was done working with my 4S today when i power it up only one channel was working then i opened it up only to find out one of the pot's(ALPS 250K) solder lug broken off i ordered a replacement i just don't know how long it would take for it to arrive.So for now i would like to use the ALPS 100K that i have on hand,My question is if i'am putting the 100K on the input and install 250K resistor on the output do i need to removed the 10K and 470K input resistors?Sorry for being a noob!

ThanksAlex

Author:

Suncalc [ 04 Jun 2017, 17:08 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

Alex Q wrote:

My question is if i'am putting the 100K on the input and install 250K resistor on the output do i need to removed the 10K and 470K input resistors?

Yup. The unit will work just fine with the 100kΩ potentiometer on the input. The volume control resistance between the wiper and ground serves as the grid resistance and the impedance seen by the grid means a grid stopper is not required. All you need is the potentiometer.

Author:

Alex Q [ 04 Jun 2017, 18:34 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

Suncalc wrote:

Alex Q wrote:

My question is if i'am putting the 100K on the input and install 250K resistor on the output do i need to removed the 10K and 470K input resistors?

Yup. The unit will work just fine with the 100kΩ potentiometer on the input. The volume control resistance between the wiper and ground serves as the grid resistance and the impedance seen by the grid means a grid stopper is not required. All you need is the potentiometer.

THANKS AGAIN!

Alex

Author:

franck.forster [ 22 Jun 2017, 17:48 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

Thanks to mwhouston and Chris65 for your helpful reply, sorry about the late reply. I had to do some works elsewhere and did not connect. I am going to start the project with the two 24 V trannies that I salvaged previously. Cheers

Author:

clowkoy [ 25 Jun 2017, 17:15 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

Hello Matt,I have a small transformer which was intended for the Decware Zkit4, 125-0 at 15 ma and 12.6 at 0.3A.If I make a voltage doubler out of this, can it support the current requirement of the 4S preamp?Thank you.

Author:

Suncalc [ 25 Jun 2017, 22:35 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

clowkoy wrote:

I have a small transformer which was intended for the Decware Zkit4, 125-0 at 15 ma and 12.6 at 0.3A.If I make a voltage doubler out of this, can it support the current requirement of the 4S preamp?

I don't see why not. Make sure you us UF diodes for the ss voltage doubler.

Author:

clowkoy [ 26 Jun 2017, 07:34 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

Thank you Matt!

Author:

Alex Q [ 26 Jun 2017, 16:53 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

I really like this preamp so i decided to build 2 of them the only issue i have for both of them is if i turn the volume up it start to hum (around 12 o'clock position) and as i turned it all the way the hum goes louder and at full turn the hum is less (so the first half turn is dead quiet) and the volume is sensitive(for both pre)i can only turn it say quarter turn and it is loud the TWO preamp have the same issue. This first Preamp have 2 inputs and one output(The VOLTAGE reading from the 100uf cap is 268VDC,From 6CA4 Pin 1 and 7 is 409VAC)

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The second Preamp have 2 inputs and 4 outputs and i used Hammond 369EX,SOLEN 47uf/630V,SOLEN 110uf/400V and CLARITY 0.47uf/630V caps i just don't want to wait for a few weeks for the right caps value that is why i used whatever is available.Is their any issue by substituting the CAPS with higher value?(The voltage reading from the 110uf cap is 270VDC,From the 6CA4 Pin 1 and 7 is 413VAC)

THANKSAlex Q

Author:

Alex Q [ 26 Jun 2017, 16:56 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

Sorry!Here is the under side of the Second Preamp.Thanks Again!Alex Q

Author:

mwhouston [ 26 Jun 2017, 18:30 ]

Post subject:

Re: 4S Universal Tube Preamp Project (12A*7)

Is the metal part of the alps at ground potential? These can buzz if not grounded.